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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
78-17058-01
Chapter11 Configuring VLANs
Creating and Modifying VLANs
VLAN Configuration Guidelines
Follow these guidelines when creating and modifying VLANs in your network:
•The switch supports 1005 VLANs.
•Normal-range Ethernet VLANs are identified with a number between 1 an d 10 01. VLA N num be rs
1002 through 1005 are reserved for Token Ring and FDDI VLANs.
•The switch does not support Token Ring or FDDI media. The switch does not forward FDD I,
FDDI-Net, TrCRF, or TrBRF t r affic.
•VLAN configurations for VLANs 1 to 1005 are always saved in the VLAN database an d in the
switch running configuration file.
•Configuration options for VLAN IDs 1006 through 4094 (extended-range VLANs) are limited to
MTU, RSPAN VLAN, private VLAN, and UNI VLAN. Extend ed-range VLANs ar e not sav ed in the
VLAN database.
•Spanning Tree Protocol (STP) is enabled by default for only NNIs on all V LANs. T he swi tch
supports 128 spanning-tree instances. If a switch has more ac tive VLANs t han s uppo rte d
spanning-tree instances, spanning tree can be enabled on 128 V L ANs a nd is di sabl ed on the
remaining VLANs. If you have already used all available spanning-tree instances on a switch,
adding another VLAN creates a VLAN on that switch that is not running spanning tree. If you have
the default allowed list on the trunk ports of that switch (which is to allow all VLANs), the new
VLAN is carried on all trunk ports. Depending on the topo lo gy o f th e net work, thi s c ould cre ate a
loop in the new VLAN that would not be broken, particularly if there are several adjacent switches
that all have run out of spanning-tree instances. You can prevent this possibility by setting allowed
lists on the trunk ports of switches that have used up their allocation of spanning-tree instances.
If the number of VLANs on the switch exceeds the number of su ppo rt ed sp an ni ng-t ree inst an ces,
we recommend that you configure the IEEE 802.1s Multiple STP (MSTP) on your switch to map
multiple VLANs to a single spanning-tree instance. For more information about MSTP, see
Chapter 15, “Configuring MSTP.”
Note MSTP is supported only on NNIs.
•Each routed port on the switch creates an internal VLAN for its use. These internal VLANs use
extended-range VLAN numbers, and the internal V LAN I D c an not be us ed for an ext end ed -ran ge
VLAN. If you try to create an extended-range VLAN with a VLAN ID that is already allocated as
an internal VLAN, an error message is generated, and the command is rejected.
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Because internal VLAN IDs are in the lower part of the extended range, we recom mend that you
create extended-range VLANs beginning from the highest nu mbe r (4 094 ) an d m oving t o the
lowest (1006) to reduce the possibility of using an internal VLAN ID.
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Before configuring extended-range VLANs, enter the show vlan internal usage privileged
EXEC command to see which VLANs have been allocated as internal VLANs.
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If necessary, you can shut down the routed port assigned to the internal VLAN, which frees up
the internal VLAN, and then create the extended-range VLAN and re-enable the port, which
then uses another VLAN as its internal VLAN. See the “Creating an Extended-Range VLAN
with an Internal VLAN ID” section on page11-11.
•Although the switch supports a total of 1005 (normal-range and extended-range) VLANs, the
number of routed ports, SVIs, and other configured features affects the use of the switch hardware.
If you try to create an extended-range VLAN and there are not eno ugh hardware resources available,
an error message is generated, and the extended-range VLAN i s r eject ed.